1. Field of the Invention
The invention relates to an image-forming device, and more specifically to a device for recording images on an image-recording medium having a dielectric surface.
2. Description of Related Art
Image-forming devices of the nature herein under consideration are described, inter alia, in European patent applications 0 191 521 and 0 304 983. As explained in these patent applications, to achieve good image quality it is important that the developing powder mass, hereinafter referred to as the "toner brush," which, in the image-forming zone, is bound by magnetic attraction to a powder support acting as the powder transport means, should have a shape substantially remaining constant. A constant brush shape is important particularly on the side where the image-recording medium leaves the image-forming zone. A constant toner brush can be produced by a magnet system of the kind described in the above-mentioned European application 0 304 983. The construction of the magnet system described therein has the disadvantage that to enable use to be made of developing powder having a low percentage of magnetic pigment by volume, it is necessary to use very strong magnets in order to achieve an adequately strong magnetic field in the image-forming zone via the knife blade. The strong magnets required, which have to be used in duplicate, one on each side of the knife blade, make the magnet system expensive and bulky. In addition, the device has the disadvantage that if toner powder is used which contains only a little magnetic pigment, there is a fairly low background-free level. The term "background-free level" denotes the maximum voltage across the image-forming zone at which no toner deposition on the image-recording medium is observed. The disadvantage of a low background-free level is that, for example, as a result of a low tribo-electric charging effect between the toner brush and the dielectric surface of the image-recording medium, the finer toner particles from the toner brush are deposited on the image-recording medium, so that images are recorded with a background.